Counterflow heat exchanger

ABSTRACT

A counterflow heat exchanger having a plurality of heat exchanger elements in a stacked, spaced-apart arrangement forming a plurality of inter-element passages. Each element has fluid passages formed between facing sections of a first plate and a second plate. A first-fluid passage has an outer passage circumscribing an inner passage in fluid communication with the outer passage. An inlet-port traversing passage in fluid communication with the outer passage, an outlet-port traversing passage in fluid communication with the inner passage and a second-fluid traversing passage circumscribed by the inner passage traverse the first and second plates. A first-fluid inlet header includes the inlet-port traversing passage of each element. A first-fluid outlet header includes the outlet-port traversing passage of each element. A second-fluid inlet passage in fluid communication with the plurality of inter-element passages includes the second-fluid traversing passage of each element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Provisional Patent Application No.61/354,943, filed Jun. 15, 2010, and incorporated herein by referenceand claims the earlier filing date of the provisional application.

BACKGROUND OF THE INVENTION

The present invention relates to a device and method for transferringheat from flue (or combustion) gas through an interposing wall (orplate) to a fluid without allowing the flue gas and fluid to mix. Moreparticularly, the present invention relates to a counterflow heatexchanger and method for cooling flue gas below a condensingtemperature.

Most fossil fuels are combusted with ambient air in a chamber, such as aboiler. The combustion product gas is exhausted from the chamber througha flue. Typical flue gas from the combustion of fossil fuels containssubstantial amounts of uncombusted nitrogen, and to a lesser degreecarbon dioxide and water vapor respectively formed by the combustion ofcarbon and hydrogen with atmospheric oxygen. In volume, the water vaporcan be as much as seven to eleven percent of the flue gas. The watervapor contains energy in the form of latent heat.

Absent a cost effective device and method for cooling the hot flue gasbelow the condensing temperature of the water vapor to recover thelatent heat and to transfer the energy in the hot flue gas to a workingfluid, the flue gas energy would be exhausted through the flue andwasted. Accordingly, a devise able to cost effectively recover latentheat from hot flue gases is desirable.

BRIEF SUMMARY OF THE INVENTION

Briefly stated, one embodiment of the present invention is directed to acounterflow heat exchanger comprising a plurality of heat exchangerelements in a stacked, spaced-apart arrangement forming a plurality ofinter-element passages. Each element comprises a first plate adjacent asecond plate. A first-fluid passage is formed between facing sections ofthe first plate and the second plate. The first-fluid passage comprisesan outer passage circumscribing an inner passage in fluid communicationwith the outer passage. An inlet-port traversing passage traverses thefirst and second plates. The inlet-port traversing passage is in fluidcommunication with the outer passage. An outlet-port traversing passagetraverses the first and second plates. The outlet-port traversingpassage is in fluid communication with the inner passage. A second-fluidtraversing passage traverses the first and second plates. Thesecond-fluid traversing passage is circumscribed by the inner passage. Afirst-fluid inlet header comprises the inlet-port traversing passage ofeach element. A first-fluid outlet header comprises the outlet-porttraversing passage of each element. A second-fluid inlet passagecomprises the second-fluid traversing passage of each element. Thesecond-fluid inlet passage is in fluid communication with the pluralityof inter-element passages.

Another embodiment of the present invention is a counterflow heatexchanger element comprising a first plate adjacent a second plate. Afirst-fluid passage is formed between facing sections of the first plateand the second plate. The first-fluid passage comprises an outer passagecircumscribing an inner passage in fluid communication with the outerpassage. An inlet-port traversing passage traverses the first and secondplates. The inlet-port traversing passage is in fluid communication withthe outer passage. An outlet-port traversing passage traverses the firstand second plates. The outlet-port traversing passage is in fluidcommunication with the inner passage. A second-fluid traversing passagetraverses the first and second plates. The second-fluid traversingpassage is circumscribed by the inner passage.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofpreferred embodiments of the invention, will be better understood whenread in conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings embodimentswhich are presently preferred. It should be understood, however, thatthe invention is not limited to the precise arrangements andinstrumentalities shown.

In the drawings:

FIG. 1 is a perspective view of the front, top and right side of apreferred embodiment of a counterflow heat exchanger in accordance withthe present invention;

FIG. 2 is a partial exploded perspective view of the counterflow heatexchanger of FIG. 1;

FIG. 3 is an assembly diagram of the counterflow heat exchanger of FIG.1;

FIG. 4 is a front perspective view of a heat exchanger element of thecounterflow heat exchanger of FIG. 1;

FIG. 5 is a side perspective view of a transfer seal of the counterflowheat exchanger of FIG. 1;

FIG. 6 is an enlarged cross sectional view of the counterflow heatexchanger of FIG. 1;

FIG. 7 is an enlarged partial view of the cross sectional view of thecounterflow heat exchanger of FIG. 6;

FIG. 8 is a schematic diagram of a cross sectional view of thecounterflow heat exchanger of FIG. 1;

FIG. 9 is a schematic diagram of a heat exchanger element of thecounterflow heat exchanger of FIG. 1; and

FIG. 10 is a front perspective view of another embodiment of acounterflow heat exchanger element in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to embodiments of the invention,examples of which are illustrated in the accompanying drawings. Theterminology used in the description of the invention herein is for thepurpose of describing particular embodiments only and is not intended tobe limiting of the invention.

As used in the description of the invention and the appended claims, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. The words“and/or” as used herein refers to and encompasses any and all possiblecombinations of one or more of the associated listed items. The words“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof.

The words “right,” “left,” “lower” and “upper” designate directions inthe drawings to which reference is made. The words “inwardly” and“outwardly” refer to directions toward and away from, respectively, thegeometric center of the structure to which reference is made, anddesignated parts thereof. The terminology includes the words notedabove, derivatives thereof and words of similar import.

Although the words first, second, etc., are used herein to describevarious elements, these elements should not be limited by these words.These words are only used to distinguish one element from another. Forexample, a first passage could be termed a second passage, and,similarly, a second passage could be termed a first passage, withoutdeparting from the scope of the present invention.

The following description is directed towards various embodiments of acounterflow heat exchanger in accordance with the present invention.

Referring to the drawings in detail, where like numerals indicate likeelements throughout, there is shown in FIGS. 1-9 a preferred embodimentof the counterflow heat exchanger generally designated 10, andhereinafter referred to as the “heat exchanger” 10 in accordance withthe present invention. In a preferred application, the heat exchanger 10is for cooling flue (or combustion) gas below the condensing temperatureof water vapor contained in the flue gas. In other applications, theheat exchanger 10 may used for cooling the hot fluid without producing acondensate.

Referring to FIGS. 1-3 and 6, the heat exchanger 10 comprises an outergas-tight wrapper 12 housing a plurality of heat exchanger elements 100in a stacked, spaced-apart arrangement 14 secured between a firstsupport plate 16 and a second support plate 18 connected by a pluralityof fasteners 20 such as tie rods with threaded ends and correspondingnuts.

In the illustrated embodiment, the outer wrapper 12 is a separable twopiece shell having a generally rectangular cross-sectional shape forconformance with the corresponding rectangular shape of the supportplates 16, 18. In another embodiment (not shown), the outer wrapper 12may have a cylindrical cross-sectional shape for conformance withcircular support plates (not shown). In general, the outer wrapper 12may have any desired shape allowing the outer wrapper 12 to form agas-tight enclosure housing the arrangement 14 of heat exchangerelements 100.

In the illustrated embodiment, the outer wrapper has an upper section 22with an exhaust port 24 and a lower section 26 with a condensate drain28 extending from a fluid collector (or trap) 30 formed in the bottom ofthe lower section 26. In an alternate embodiment, the trap 30 may be acomponent separate from the outer wrapper 12 and part of a drainagesystem to which the outer wrapper is plumbed.

In another embodiment (not shown), the outer wrapper 12 may be more thantwo sections or may be separable into front and rear sections instead ofupper and lower sections depending on the form factor desired for aparticular installation.

The edges of the upper and lower sections 22, 26 forming the outerwrapper 12 are configured to join each other and the first (or front)support plate 16 and the second (or rear) support plate 18 in a mannerable to form a gas tight enclosure for the arrangement 14 of heatexchanger elements 100 enclosed therein. For example, the outer edges ofthe upper and lower sections 22, 26 of the outer wrapper 12 may beoutwardly flanged to join the sections to each other. Other outer edgesmay be inwardly flanged to join the first and second support plates 16,18 to the upper and lower sections 22, 26 of the outer wrapper 12. Theflanges may be channeled to receive or support a gasket or elastomericmaterial between adjoining surfaces and to receive a fastener releasablyjoining the component parts.

The outer wrapper 12 may be fabricated from a wide variety of heatresistant materials including, but not limited to, polyphenylenesulfide, glass-filled polypropylene or similar high-performancethermoplastics. The outer wrapper 12 may also be fabricated from coatedsteel or high grade stainless steel.

Referring to FIGS. 2-4, in one embodiment, the heat exchanger elements100 comprising the arrangement 14 are formed by circular plate pairs.Each element 100 comprises a first plate 102 adjacent a second plate104. A first-fluid passage 106 is formed between facing sections of thefirst plate 102 and the second plate 104 by opposing channels in thefacing sections. The first-fluid passage 106 comprises an outer passage108 circumscribing an inner passage 110 in fluid communication with theouter passage through one or more interconnecting passages 112. In someembodiments, the outer passage 108 may be spaced from the inner passage110 by a web 109 through which the interconnecting passages 112 pass. Aninlet-port traversing passage 114 in fluid communication with the outerpassage 108 traverses the first and second plates 102, 104. In someembodiments, each element 100 may have additional port-traversingpassages, such as a second (or alternate) port traversing passage 114_(a), to provide the ability to accommodate alternate uses and plumbingconfigurations. For example, the alternate port traversing passage 114_(a) may be used in some embodiments as an additional inlet-porttraversing passage or in other embodiments as a drain port. Anoutlet-port traversing passage 116 in fluid communication with the innerpassage 110 also traverses the first and second plates 102, 104. Inaddition to the inlet-port and outlet-port traversing passages 114, 116,the first and second plates 102, 104 of each element 100 are traversedby a centrally-positioned, second-fluid traversing passage 118circumscribed by the inner passage 110.

In some embodiments, the plates forming each element 100 may have agenerally circular shape and may have concentric outer and innerpassages as shown in FIG. 4. In other embodiments, the plates may have asquare or rectangular shape such as the heat exchanger element 100′shown in FIG. 10. In general, the plates may have any desired shapeallowing the outer wrapper 12 to form a gas-tight enclosure housing thearrangement 14 of elements 100.

Referring to FIG. 10, in some embodiments, a first-fluid passage 106′ ofeach heat exchanger element 100′ may have one or more generallyconcentric fluid passages 106 _(n)′ between the outer and inner passages108′, 110′. Interconnecting passages 112′ provide a fluid path betweenthe one or more concentric fluid passages 106 _(n)′, the outer passage108′ and the inner passage 110′. The interconnecting passages 112′ maybe radially oriented or may be angled with respect to a radius. Otherembodiments may have more than three concentric passages or less thanthree concentric channels with one or more interconnect passagesconnecting adjacent concentric passages. In still other embodiments, thepassages need not be concentric channels. Although circumscribing eachother, the passages may have a generally meandering shape. Each heatexchanger element 100′ may also have an inlet-port traversing passage114′, outlet-port traversing passage 116′ and a centrally-positioned,second-fluid traversing passage 118′ circumscribed by the inner passage110′.

In some embodiments, the outwardly facing surfaces of each element mayhave raised, radially-oriented dimples 120 arranged and may beconfigured to allow generally uniform radial flow of a fluid over thesurfaces (See, e.g., FIG. 4). In other embodiments, the outwardly facingsurfaces of each element may be smooth. (See, e.g., FIG. 10.)

The material from which the heat exchanger elements are fabricated ispreferably a metal such as 316 stainless steel. Alternatively, 304 or400 series stainless may be used. Other thermally conductive materials,titanium for example, compatible with the combustion gas products andthe cooling (or working) fluid may also be used. The channels in thefirst and second plates 102, 104 forming the passages in each element100, 100′ are preferably created by stamping or hydroforming.Alternatively, the plates and the channels therein may be formed bycasting.

Referring to FIGS. 2, 3 and 6, the stacked, spaced-apart arrangement 14into which the plurality of heat exchanger elements 100 are assembledhas a first-fluid inlet header 32 comprising the inlet-port traversingpassage 114 of each element 100 and a first-fluid outlet header 34comprising the outlet-port traversing passage 116 of each element 100.In some embodiments, a plurality of transfer seals 36 described belowcouple the inlet-port traversing passages 114 comprising the first-fluidinlet header 32 and the outlet-port traversing passages 116 comprisingthe first-fluid outlet header 34. The plurality of heat exchangerelements 100 are spaced-apart by the plurality of transfer seals 36forming a plurality of inter-element passages 38 in the assembly 14.

Referring to FIGS. 3 and 5-7, in some preferred embodiments, eachtransfer seal 36 may be a generally cylindrical component with a centralpassage 40 extending axially therethrough. The transfer seal 36 may havea generally cylindrical central portion 42 configured to make sealingcontact with one of the inlet-port traversing passages or outlet-porttraversing passages when the plurality of heat exchanger elements areassembled into the spaced-apart arrangement. A nipple 44 extends axiallyfrom each side of the generally cylindrical central portion 42. Eachnipple 44 has a nipple outer diameter sized for insertion in one of theinlet-port traversing passages 114 or outlet-port traversing passages116. In some embodiments, the nipple outer diameter may be less than theinner diameter of the inlet-port or outlet-port traversing passages 114,116 allowing each nipple to be inserted in the traversing passages witha clearance fit when the heat exchanger elements 100 are assembled toform the spaced-apart arrangement 14. Alternatively, in otherembodiments, the nipple outer diameter may be greater than the innerdiameter of the inlet-port or outlet-port traversing passages 114, 116requiring each nipple to be inserted in the traversing passages with aninterference (or press) fit. In some embodiments, the outer diameter ofthe central portion 42 of each transfer seal 36 may be sized to begreater than the outer diameter of the nipples 44. In other embodiments(not shown), the outer diameter of the central portion 42 of eachtransfer seal 36 may be sized to be less than or equal to the outerdiameter of the nipples 44. In some embodiments, a circumferentialO-ring channel 46 in the outer surface of each nipple 44 retains anO-ring 48 sized to make sealing contact between the nipples 44 and oneof the inlet-port traversing passages 114 or outlet-port traversingpassages 116 when the elements 100 are assembled into the arrangement14. The generally cylindrical central portion 42 of the transfer seal 36may have an axial extent sufficient to provide a desired predeterminedspacing for the inter-element passages 38 formed between the pluralityor heat exchanger elements 100 of the spaced-apart arrangement 14.

The transfer seals 36 may be fabricated from the same materialidentified above for the plates 102, 104 forming the each element 100.The O-ring 48 may be made from any elastomer compatible with flue gasproducts and is preferably made for ethylene propylene diene monomer(M-class) rubber or Viton™, a brand of synthetic rubber andfluoropolymer elastomer made by DuPont Performance Elastomers L.L.C.

When the plurality of heat exchanger elements 100 is assembled togetherwith the transfer seals 36 into the arrangement 14 in accordance withFIG. 3, a second-fluid inlet passage 50 comprising the second-fluidtraversing passage 118 of each element 100 is formed. The second-fluidinlet passage 50 is in fluid communication with the plurality ofinter-element passages 38.

The second-fluid inlet passage 50 has a first end 50 a coupled to asecond-fluid inlet port 52 in the first support plate 16. In someembodiments, the second end 50 b of the second-fluid inlet passage 50may be coupled to a removable closure 54 configured to seal thesecond-fluid traversing passage 118 of a last element of the pluralityof heat exchanger elements 100 in the stacked arrangement 14.

In some embodiments, the stacked arrangement 14 of heat exchangerelements 100 is secured between the first support plate 16 and thesecond support plate 18 by a plurality of fasteners 20 connecting thetwo plates 16, 18. For ease of assembly and disassembly, the fasteners20 preferably are tie rods that terminate in a threaded portion forreceiving a threaded nut and extend between and through penetrators 58in the first and second support plates 16, 18.

In some embodiments (not shown), the first and last elements of theplurality of elements 100 may replace the first and second supportplates 16, 18. In such embodiments, each element, including the firstand last elements, may have penetrators (see, e.g., FIG. 10) throughwhich the tie rods pass. Further, the last element may not have asecond-fluid traversing passage. Instead, the central area circumscribedby the inner passage 110 remains a continuous web. The internal surfaceof the last element may or may not be provided with a thermallyinsulating insert.

In some embodiments, the second-fluid inlet port 52 may traverse thefirst support plate 16 and the second support plate 18 may have afirst-fluid inlet port 60 and a first-fluid outlet port 62. When theplurality of heat exchanger elements 100 are housed in an enclosureformed by the outer wrapper 12, the first-fluid inlet port 60 is influid communication with the first-fluid inlet header 32 and thefirst-fluid outlet port 62 is in fluid communication with thefirst-fluid outlet header 34. The second-fluid inlet port 52 is in fluidcommunication with the second-fluid inlet passage 50. The exhaust port24 is in fluid communication with the inter-element passages 38. Thecondensate drain 28 also is in fluid communication with theinter-element passages 38.

During operation, the internal pressure in the inlet and outlet fluidheaders 32, 34 acts circumferentially on the internal surface of thenipples 44. Since the internal pressure serves to reinforce the radialsealing surface, the transfer seals 36 do not depend on the tie rods 20to compress the O-rings 48. The stress in the tie rods 20 ispredominantly induced by the internal pressure of the working (or first)fluid.

In one embodiment, the heat exchanger 10 may be designed to be used inconjunction with a gas or liquid fuel combustion system, such as a gasor oil burner. In another embodiment, the heat exchanger 10 may bedesigned to be used with a solid fuel burner or other source of hotgases. The method for manufacturing the heat exchanger 10 and thematerials from which the heat exchanger elements are fabricated may beapplication specific. For example, the elements could be stamped fromaluminum or stainless steel for condensing boilers, or from steel fornon-condensing boilers. Alternatively, the heat exchanger elements couldbe die-cast aluminum for condensing boilers and die-cast steel fornon-condensing boilers or they could be cast aluminum for condensingboilers or cast iron for non-condensing boilers.

FIGS. 8 and 9 are representative flow diagrams of the heat exchanger 10in use. Combustion gases enter the center cylindrical opening or secondfluid inlet passage 50 through a gas flue (not shown) attached to thesecond-fluid inlet port 52 in the first support plate 16. The gases flowaxially through the second-fluid inlet passage 50 and radially outwardlythrough the inter-element passages 38 between the elements 100 of thearrangement 14. As the gases flow radially outwardly, the gasessuccessively contact the concentric fluid passages (e.g., the innerpassage 110 and then the outer passage 108) comprising the first-fluidpassage 106, and flow over and in contact with the web 109 which acts asan extended heating surface between the concentric fluid passages. Whenthe gases flow beyond the extent of the arrangement 14, the gasses arecaptured by the outer, gas-tight wrapper 12 and exit through the exhaustport 24.

In one embodiment a first fluid (e.g., a coolant or working fluid) froma heating system enters the inlet header 32 of the heat exchanger 10through the first-fluid inlet port 60 of the second support plate 18.The working fluid travels radially inwardly in a counter flow directionto the flow of the combustion gasses from the outermost concentric fluidpassage 108 through the interconnecting passage 112 (and interveningconcentric fluid passage ways, if any,) to the innermost concentricpassage way 110 and into the outlet fluid header 34. The working fluidexits the heat exchanger 10 through the first-fluid outlet port 62 inthe second support plate 18 and returns to the heating system.

As the combustion gas and the working fluid cross-flow through the heatassembly 14, water vapor in the combustion products condenses. Thelatent heat released during condensation transfers to the plurality ofheat exchanger elements 100 increasing the temperature of the workingfluid. The condensate drops to the bottom of the outer wrapper 12 andexits the heat exchanger 10 through the condensate drain 28 in the outerwrapper 12.

The transfer of energy to the working fluid and the rise in thetemperature of the working fluid is diagrammatically represented by thechange in the stippling of the working fluid from lightly stippled (arelatively cool inlet temperature) to more heavily stippled (arelatively hot outlet temperature) as the working fluid flows radiallyinwardly from the outer passage 108 to the inner passage 110.

In typical combustion system applications, the following operationconditions may occur. The maximum internal pressure may be 160 psig andthe maximum temperature may be 210-250° F. The liquid flow rates mayvary with inlet rate to develop a 10-100° F. temperature differenceacross the heat exchanger. The gas flow rates are sufficient to providereasonable combustion at rated inlet (Gas CFH×9.5×1.5). Combustion sidepressure is usually less than 14 in of water but could be more.

The above disclosed embodiments may be modified to accommodate a widerange of operating conditions more or less demanding than the conditionsset forth above. For example, in some embodiments, the number of heatexchanger elements may be increased or decreased to accommodate greateror lesser thermal loads. Further, the elements may be designed to havemore or less passages (having a concentric or other circumscribingshape) to allow scaling of the design to larger or smaller sizes. Stillfurther, although the embodiments disclosed above were directed tocombustion systems, the heat exchanger 10 is not limited to combustionapplication. Rather, the heat exchanger 10 may be implemented in variousembodiments suitable for use in instances where there is a desire tocapture the latent energy associated with the condensation of one ormore vapors in a gas stream or, stated more generally for non-condensingapplications to capture some of the thermal energy in a gas stream.Therefore, the invention disclosed above is not limited to theparticular embodiments or applications disclosed. Rather, the disclosureis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A counterflow heat exchanger comprising: a plurality of heatexchanger elements in a stacked, spaced-apart arrangement forming aplurality of inter-element passages, each element comprising: a firstplate adjacent a second plate; a first-fluid passage formed betweenfacing sections of the first plate and the second plate, the first-fluidpassage comprising an outer passage circumscribing an inner passage influid communication with the outer passage; an inlet-port traversingpassage traversing the first and second plates, the inlet-porttraversing passage in fluid communication with the outer passage; anoutlet-port traversing passage traversing the first and second plates,the outlet-port traversing passage in fluid communication with the innerpassage; and a second-fluid traversing passage traversing the first andsecond plates, the second-fluid traversing passage circumscribed by theinner passage; a first-fluid inlet header comprising the inlet-porttraversing passage of each element; a first-fluid outlet headercomprising the outlet-port traversing passage of each element; and asecond-fluid inlet passage comprising the second-fluid traversingpassage of each element, the second-fluid inlet passage in fluidcommunication with the plurality of inter-element passages.
 2. Thecounterflow heat exchanger according to claim 1, wherein the outerpassage, the inner passage and the second fluid passage are concentric.3. The counterflow heat exchanger according to claim 1, wherein eachelement has a generally circular shape and the outwardly facing surfacesof each element have raised, radially-oriented dimples arranged andconfigured to allow generally uniform radial flow of a fluid over thesurfaces.
 4. The counterflow heat exchanger according to claim 1,further comprising a plurality of transfer seals having a centralpassage therethrough, the plurality of transfer seals coupling theinlet-port traversing passages comprising the first-fluid inlet headerand the outlet-port traversing passages comprising the first-fluidoutlet header.
 5. The counterflow heat exchanger according to claim 4,wherein each transfer seal comprises: a generally cylindrical centralportion configured to make sealing contact with one of the inlet-porttraversing passages or the outlet-port traversing passages when theplurality of heat exchanger elements are assembled into the spaced-apartarrangement.
 6. The counterflow heat exchanger according to claim 5,wherein each transfer seal further comprises: a nipple extending axiallyfrom each side of the generally cylindrical central portion; each nipplehaving a nipple outer diameter sized for insertion in one of theinlet-port traversing passages or the outlet-port traversing passageswith a clearance fit; the central portion of each transfer seal having atransfer seal outer diameter sized to be greater than the nipple outerdiameter; a circumferential O-ring channel in an outer surface of eachnipple; an O-ring retained in the O-ring channel and sized to makesealing contact between the nipple and one of the inlet-port traversingpassages or the outlet-port traversing passages when the plurality ofheat exchanger elements are assembled into the spaced-apart arrangement.7. The counterflow heat exchanger according to claim 5, wherein thegenerally cylindrical central portion of each transfer seal has an axialextent sufficient to provide a predetermined spacing for theinter-element passages formed between the plurality of heat exchangerelements of the spaced-apart arrangement.
 7. The counterflow heatexchanger according to claim 1, wherein the plurality of heat exchangerelements are spaced-apart by a plurality of transfer seals coupled tothe inlet-port traversing passages and the outlet-port traversingpassages.
 8. The counterflow heat exchanger according to claim 1 whereinthe plurality of heat exchanger elements are housed in an enclosurehaving a first-fluid inlet port in fluid communication with thefirst-fluid inlet header, a first-fluid outlet port in fluidcommunication with the first-fluid outlet heard, a second-fluid inletport in fluid communication with the second-fluid inlet passage, anexhaust port in fluid communication with the inter-element passages anda condensate drain in fluid communication with the inter-elementpassages.
 9. The counterflow heat exchanger according to claim 1 whereinthe stacked arrangement of heat exchanger elements is secured between afirst support plate and a second support plate connected by a pluralityof tie rods.
 10. The counterflow heat exchanger according to claim 1wherein the second-fluid inlet passage has a first end coupled to asecond-fluid inlet port and a second end coupled to a closure configuredto seal the second-fluid transfer passage of a last element of theplurality of heat exchanger elements in the stacked arrangement.
 11. Aheat exchanger element comprising: a first plate adjacent a secondplate; a first-fluid passage formed between facing sections of the firstplate and the second plate, the first-fluid passage comprising an outerpassage circumscribing an inner passage in fluid communication with theouter passage; an inlet port traversing the first and second plates, theinlet port in fluid communication with the outer passage; an outlet porttraversing the first and second plates, the outlet port in fluidcommunication with the inner passage; and a second-fluid passagetraversing the first and second plates, the second-fluid passagecircumscribed by the inner passage.
 12. The counterflow heat exchangerelement according to claim 11, wherein the outer passage, the innerpassage and the second-fluid passage are generally concentric.
 13. Thecounterflow heat exchanger element according to claim 11, wherein theelement has a generally circular shape and the outwardly facing surfacesof each element have raised, radially-oriented dimples arranged andconfigured to allow generally uniform radial flow of a fluid over thesurfaces.